This invention generally relates to a mercury free alkaline cell comprising a zinc anode. More particularly, this invention is concerned with an alkaline electrochemical cell that is capable of providing optimum service at various discharge conditions.
Commercially available cylindrical alkaline electrochemical cells are widely available in cell sizes commonly known as AA, AAA, C and D. In many cases the cells are purchased by consumers and then stored until they are needed to power a device. Due to the proliferation of battery powered devices, many consumers own numerous battery powered devices. Some of the devices that may be found in one home include: a radio; a remote control for a television set; a tape recorder; toys for children; an handheld electronic game; a compact disc player; a camera that incorporates a flashlight unit and 35 millimeter film; and a digital camera. Collectively, these devices represent a wide range of electrical discharge conditions. For example, a wall clock is known within the battery manufacturing field as a “low drain” device because it needs the battery to supply current at a very low rate and with brief rest periods between activations. Consequently, the clock may be powered for more than a year by a single AA size battery. Another device, such as a compact disc player, requires several batteries to supply current at a faster rate than is required by a clock but with substantial rest periods between activations (i.e. 250 milliamps for one hour per day) and is known as a “high tech” device. Other devices, such as a digital camera, require the battery to supply a substantial current at a high voltage (i.e. a 1000 milliwatt drain with no rest periods) and is recognized as a “high drain” device. When consumers purchase batteries, the consumer may not know the device into which a particular battery will be inserted. Consequently, the consumer will attempt to purchase batteries that perform well in a variety of devices that may incorporate low drain, high drain and/or high tech discharge conditions. If a consumer believes that a particular brand of battery provides optimum service when used in all devices, then the consumer will be motivated to buy that brand of batteries rather than a different brand of batteries. Consequently, many battery manufacturers strive to develop and market batteries that are perceived by the consumer as “all purpose” batteries because the batteries power a wide range of devices for acceptable periods of time.
In addition to improving the length of time that their products will power a variety of devices, battery manufacturers constantly strive to reduce the cost of the battery. One way to reduce the cost is to decrease the quantity of electrochemically active material in one or both of the battery's electrodes. For example, the quantity of zinc in the anode and/or the quantity of manganese dioxide in the cathode could be reduced. However, this option is not acceptable to the manufacturer because any reduction in the quantity of electrochemically active material usually decreases the battery's “run time” which is the length of time the battery will run a device. Furthermore, reducing the volume of zinc in the anode can lead to inadequate electrical conductivity within the anode thereby causing sporadic or premature failure of the cell during discharge. This problem was recognized by the manufacturers of alkaline batteries when mercury was removed from the anode in order to make the batteries more environmentally friendly upon disposal. Unfortunately, removal of the mercury caused a significant portion of the cells to display erratic electrical discharge patterns due to the loss of conductivity between the zinc particles. To correct this problem, the quantity of zinc in the anode was increased thereby insuring a conductive matrix throughout the anode. In conventional cells that contain no added mercury and only particulate zinc powder as the anode's electrochemically active material, the anodes contained no less than 28 volume percent of particle zinc powder. As used herein, the phrase “no added mercury” means that the anode contains less than 50 ppm of mercury. Preferably, the anode contains no added mercury. Therefore, there is a widely recognized need to find a way to decrease the amount of electrochemically active material included mercury free cells without adversely impacting the cell's run time.
Previous attempts to address the problem of how to improve a battery's performance in a particular device, such as a digital camera, have usually involved changes to the cell's internal construction. In one example, the cell construction was modified by increasing the quantity of zinc in the anode. However, this change resulted in unacceptable leakage of electrolyte after the cell had been deeply discharged. In another example, instead of using a cell design in which one electrode is inserted into a centrally aligned cavity defined by the other electrode, some manufacturers have used a “jellyroll” construction in which two strip shaped electrodes and one separator are aligned with one another and then rolled to form a coil. Batteries with jellyroll constructions typically perform well in high drain devices. Unfortunately, the same cells provide substantially reduced service in low drain devices because a substantial portion of the electrochemically active material must be replaced with chemically inert separator due to the jellyroll's large anode-to-cathode surface area. Consequently, batteries made with a jellyroll construction are not well suited for use in devices where the cell's total electrochemical capacity is more important than the ability to discharge at a rapid rate.
Therefore, there is a need for an inexpensive mercury free alkaline electrochemical cell that has the ability to provide adequate run times in devices that require the battery to discharge at a high drain rate as well as provide adequate run times in devices that require the battery to discharge at a low drain rate.